1. Field of the Invention
The present invention generally relates to optical recording for writing data in a phase-change optical recording medium using a sequence of recording pulses, and more particularly, to a pulse control technique for controlling the amplitude of the recording pulses when writing data in an optical disk.
2. Description of Related Art
JP 2002-216349A discloses a technique for selecting the optimum waveform of a laser pulse in response to change in the surrounding temperature, when recording data in an organic dye recording layer of an optical disk (such as DVD-R). It is pointed out in this publication that variation in wavelength is one of the factors that cause degradation of the recording quality. Variation in wavelength is detected indirectly by measuring the temperature surrounding the laser, and the optimum shape of the recording pulse is selected based on the measurement result.
In recent years and continuing, various types of optical disks are put into practical use, along with the active progress of optical technique. Examples of such optical disks include read-only optical disks (such as music CDs or CD-ROMs), writable disks using dye media, and rewritable disks (such as CD-RW) using phase-change recording media. In addition, large capacity optical disks, such as DVD-ROMs (digital versatile disks-read only memories), DVD+R, and DVD+RW, are also attracting attention. These large capacity optical disks have been realized owing to miniaturization of the spot diameter by shortening the wavelength of the semiconductor laser and employment of a high-NA objective lens.
To record data in any type of optical disk, an optical recording/reproducing apparatus with an optical pickup is used. In general, a semiconductor laser is employed as the light source in the optical pickup system. For example, in single-pulse optical recording, a modulated emission waveform is used to record data in phase-change media. The modulated emission waveform is generated using eight-to-fourteen modulation (EFM) codes or eight-to-sixteen (8-16) modulation codes.
However, with the single-pulse optical recording techniques, recording marks are likely to deform into tear-drops due to heat accumulation, and subsequent phase change to the amorphous state cannot be accomplished satisfactorily due to insufficient cooling rate. Thus, recording marks with low reflectance with respect to laser light cannot be acquired when using the single-pulse recording scheme. To overcome this problem, a multipulse recording scheme for recording data in phase change recording media using a sequence of pulse consisting of one or more short pulses is employed. The pulse emission rule, including the pulse width and the amplitude of the sequence of short pulses, is called a strategy. The strategy is determined such that the sequence of optical emission can produce a mark consistent with the modulated emission waveform generated based on EFM codes or 8-16 modulation codes. A sequence of pulses includes a front pulse (FP) for sufficiently heating the recording layer of a phase change recording medium at or above the melting point in advance, a last pulse (LP) of the heating cycle, multiple heating pulses (ML) successively generated between the front pulse and the last pulse, and a cooling pulse (CP) following the last pulse. The last pulse (LP) and the multiple pulses (ML) may be set to zero depending on the mark length. For instance, LP and MP are not used when recording 3T marks.
Using the multipulse emission waveform, mark regions of a phase change recording medium can change to the amorphous state under the rapid cooling condition, with prompt change from the heating state to the cooling state using a group of heating pulses FP, MP and LP and a cooling pulse CP. In the space regions, the crystalline phase is produced under the slow cooling condition using erase pulses, and therefore, adequate reflectance difference is acquired between amorphous phase and crystalline phase.
The recording characteristics of the phase change optical disk include, for example, jitter in the reproduction signals, overwrite capabilities, and power margins. These parameters vary depending on the pulse widths of the heating pulses (FP, MP, and LP) and the cooling pulse (CP), the recording power (emission power), the type and linear velocity of the optical disk, the surrounding temperature, etc.
In conjunction with the dependency of the recording characteristics, the above-described publication JP 2002-216349A tries to detect fluctuation in wavelength by measuring the surrounding temperature of the laser and to select the optimum recording pulse based on the measurement result for an organic dye recording medium, such as DVD-R.
This technique is designed for organic dye optical disks, in which recording marks are formed in an organic dye recording layer based on the heating duty of laser pulses, and accordingly, it cannot be applied as it is to phase-change type optical disks because in phase-change optical disks recording marks are formed by rapid phase transition to amorphous state under rapid heating and cooling processes.
Formation of the amorphous phase under rapid heating and cooling conditions is affected by change in the surrounding temperature. If the temperature surrounding the optical disk rises to be high, the recording layer is heated too much during the rapid heating process, while the cooling rate becomes insufficient in the rapid cooling process. Consequently, the recording quality is degraded.
Optical disk recording and reproducing equipment is generally assembled into a personal computer. Due to the high-speed operations and multi-function configuration of recent personal computers, the amount of heat generated by various components is increasing inside the personal computer, and the environmental temperature of the personal computer inevitably rises. Internal temperature rise is especially conspicuous in highly integrated notebook computers. If the recording strategies optimized for room temperature are applied to the high temperature environment, such as inside personal computers, the recording quality designed to be achievable at room temperature cannot be acquired.